Weeds: Knotweeds (Bohemian, Giant, Japanese, Himalayan) – Polygonum spp.

• Family: Polygonaceae
• Cycle: Perennial
• Plant type: Broadleaf

Biology

The giant knotweeds are aggressive plants that spread via long rhizomes and also by seeds. They are deciduous plants, with the woody, reddish-brown stems dying back in winter. Summer growth of Japanese knotweed or fleeceflower (Polygonum cuspidatum, also known as Fallopia japonica) can reach nine feet in height. The large leaves are truncate, somewhat heart-shaped to broadly oval, with pointed tips. The leaf blades range from two to six inches long and from two to four inches wide. The underside may have barely noticeable bump-like hairs, especially along the midvein. Leaves are borne on short petioles, alternating along the slightly zigzagging, jointed stems. Flowers are borne in cream-colored or greenish-white clusters in the leaf axils, appearing in late summer to fall. Giant knotweed or sachaline (Polygonum sachalinense, also known as Fallopia sachalinensis) is a larger (up to twelve feet in height) but otherwise similar species which is found west of the Cascades. Its leaves are about three times as large as those of Japanese knotweed, and have a more distinct heart-shaped appearance. Hairs on the leaf undersides are long and wavy. Bohemian knotweed (Polygonum x bohemicum) is a cross between Japanese knotweed and giant knotweed and has some characteristics of both parents. Leaves tend to be heart-shaped near the base of the stems and more spade-shaped on the upper portions of the stems. Leaf hairs are short and broad-based, intermediate between the parent species. Himalayan knotweed (P. polystachyum) is a smaller species (up to six feet in height) and has more oblong, lance-shaped leaves and white/pink flower clusters. SPECIAL INFORMATION: The giant knotweeds are very aggressive plants, spreading rapidly to become a problem weed in some areas. In WASHINGTON all four species are designated as Class ‘B’ noxious weeds. In OREGON, all except Bohemian knotweed are designated as Class ‘B’ noxious weeds. Management of these species may be required by law in your county. In addition, giant knotweeds are on the noxious weed quarantine list for both Washington and Oregon. Sale, purchase, and transport of plants, plant parts, and seeds is prohibited. Consult your local Noxious Weed Control Board for more information.

Habitat

The giant knotweeds are escaped ornamental species that may be found on waste places, roadsides, ditch banks, and other sites. Japanese and giant knotweed grow best in sunny, moist areas and can tolerate heavy soils. Himalayan knotweed, however, grows on most soil types and in both sunlight and partial shade.

Management Options

Use Integrated Pest Management (IPM) for successful plant problem management.

Non-chemical Management

Select non-chemical management options as your first choice!

• None recommended

Chemical Management

IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides.

• Apply according to label directions.
• Treatment is the same for all of the giant knotweeds.
• Spot treatments with certain post-emergent herbicides will control weedy grasses, but will also kill the turf.
• Glyphosate products should be applied as spot treatments only!
• NOTE: Some ingredients listed here are only available in combination.
• Read the label carefully on combination products to make sure the product is suitable for your specific situation.

Additional Images